The present invention relates to a polyester film for a decorative plate or decorative sheet. More particularly, the present invention relates to a polyester film for a decorative plate laminated by clear film, which comprises a coated polyester film having specific properties, and shows an excellent suitability for decorative plate.
The decorative plate has a laminated structure in which at least a film layer and a picture-printed layer are in turn superimposed over a surface of substrate, and generally means a building material (surface material) capable of being dealt as an independent product. The decorative sheet means a decorative material (surface material) which is adhered to a surface of substrate for various products such as furniture, building components or housing appliances and provided on a surface thereof with a picture-printed layer. Accordingly, the film layer and the picture-printed layer laminated over the surface of substrate of the decorative plate, are components of the decorative sheet.
As a sheet material interposed between the substrate and the picture-printed layer of the decorative plate or a sheet material (film) used in the decorative sheet, vinyl chloride resin sheets are most generally used. However, in the case where the vinyl chloride resin sheet is used, there arise such problems that a plasticizer blended in the sheet material is transferred to an adhesive layer formed on its surface to be adhered, resulting in deteriorated adhesion thereof, or that the vinyl chloride resin sheet suffers from elongation or shrinkage by heating due to low thermal dimensional stability thereof, thereby causing wrinkles thereon. Further, in recent years, there has been an increasing demand for producing decorative plates or decorative sheets without using the vinyl chloride resin sheet, from a standpoint of environmental protection.
Meanwhile, as described above, since decorative plates and decorative sheets are used as a surface material, picture patterns having a high design value are generally formed on a surface thereof. Therefore, it is extremely important to adjust or control color tones of the decorative plates or decorative sheets in order to obtain appropriate contrasts between the picture patterns or delicate contrasts of shade and shadow thereon. Specifically, as the substrates of the decorative plates or the substrates to which decorative sheets are attached, various materials such as plywood, e.g., particle boards, steel boards or the like have been used. In general, these substrates have a variety of color tones which are different from each other, even if they are formed from the same material.
Accordingly, if individual films for the decorative sheet have a deteriorated hiding power, the color tone of the substrate adversely affects that of the surface of the decorative plate or decorative sheet, so that inherent high design value of the picture patterns formed thereon are damaged. For this reason, it is required that the sheet materials or films for the decorative plates or decorative sheets have a high hiding power, thereby enabling these sheet materials or films to be widely applicable to substrates having various color tones.
On the other hand, biaxially-oriented polyester films, typically polyethylene telephthalate films have been widely utilized in various fields because of their excellent properties. As disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 7-17005, these biaxially-oriented polyester films have been proposed so as to be used only as surface materials for decorative plates. However, there has not yet been any proposal in which the biaxially-oriented polyester films are interposed between the substrate and the picture printed layer, in other words, that the biaxially-oriented polyester films are used as a sheet material or film for decorative sheets.
Meanwhile, polyester films are generally deteriorated in adhesion to various materials as compared with vinyl chloride resin films or sheets. Accordingly, in the case where the polyester films are used instead of the vinyl chloride resin films or sheets, there are caused disadvantages, e.g., deteriorated adhesion to wood as the substrate. The decorative plates are used to decorate surfaces of various furniture, doors or the like. Therefore, the occurrence of delamination or separation of the film from the substrate should be prevented because the delamination or separation shows that the decorative plates are imitation products.
As a material for a sash bar for receiving an aluminum sash, in many cases, there have been used decorative plates composed of a wooden material. In this case, the decorative plates used as the sash bar are exposed to severe change in temperature between summer and winter seasons together with load of the aluminum sash. Under such a circumstance, if the adhesion between the film and the substrate is deteriorated, the delamination or separation of the film from the substrate is likely to occur, thereby causing undesirable mountain range-like undulations along the sash bar. In addition, when it is intended to form picture patterns having a high design value on the film by printing or the like, there arise problems such as deteriorated suitability for printing, deteriorated wettability for ink or adhesive, occurrence of delamination or separation of the laminated printed layer due to low adhesion thereof.
Further, with respect to the decorative plates laminated by clear film, there arises the below-mentioned problems.
Decorative plates having such a laminated structure in which an adhesive layer, a film layer and a picture printed layer are in turn superimposed on a substrate are called "printed decorative plate". The decorative plates having such a laminated structure in which a first adhesive layer, an inner film layer, a picture printed layer, a second adhesive layer and an outer film layer are in turn superimposed on a substrate are called "decorative plate laminated by clear film". Incidentally, as the substrates, wood-based boards such as plywood or particle boards, inorganic boards, steel boards or the like can be appropriately used.
Especially, the decorative plate laminated by clear film has such technical advantages that (i) an appropriate protection for the picture-printed layer can be attained by the outer film layer, (ii) when the outer film layer comprises a transparent film, the decorative plate can exhibit a high-grade picture pattern, and (iii) when the outer film layer comprises an embossed film, the decorative plate with a three-dimensional moire finishing can be produced. For these reasons, the decorative plates laminated by clear film can be used in higher-grade applications as compared with the printed decorative plates.
Owing to the afore-mentioned advantages, the decorative plates laminated by clear film can be applied to front face decorations of various furniture, doors or the like. Especially, in the case of furniture, doors or the like comprising a wood-based board, the corners thereof are defined by adjacent two flat portions which are crossed at a right angle. These corners can be formed by bending the afore-mentioned decorative plate.
The bending of the decorative plate has been generally performed by a so-called V-cutting process. In the V-cutting process, V-shaped notches are formed on a surface of a substrate of the decorative plate such that the depth of each notch is slightly smaller than the thickness of the substrate. The decorative plate is then bent at these V-shaped notches such that the notched surface of the decorative plate is located inside. For this reason, the suitability for bending deformation process, of the decorative plate is highly relied upon those of the films used therein. Especially, in the case of the decorative plates laminated by clear film in which inner and outer films are used, it is required that these films show more excellent suitability for bending deformation process as compared with in the case of the printed decorative plate having only one film. If the films are deteriorated in suitability for bending deformation process, the bent shape of the decorative plate cannot be kept stable when subjected to V-cutting and bending processes, but readily returns back to its original flat shape. Further, at this time, there is a likelihood in which the decorative sheet comprising the inner film layer, the picture-printed layer, the adhesive layer and the outer film layer is separated from the substrate, so that the decorative plate is deteriorated in its appearance or design value. Therefore, it is required that the films used in the decorative plate laminated by clear film shows excellent suitability for V-cutting and bending processes.
Also, in the case where the decorative plate is used as surface materials for furniture, doors or the like, it is inevitably brought into contact with, for example, tip ends of a cleaner, nails or the like. If the films adhered to the substrate of the decorative plate have low self-healing properties (restoring properties), dents caused thereon by the afore-mentioned contact are left as they are, so that a high design value of the picture pattern of the picture-printed layer cannot be maintained. In addition, if the films adhered to the substrate of the decorative plate do not have an appropriate strength or rigidity, plastic deformation thereof becomes increased and it is impossible to remove such dents by self-healing. Especially, in the case of the decorative plate laminated by clear film, it is required that the films to be adhered to the substrate of the decorative plate are excellent in self-recovery for dent deformation (resistance to impact deformation) which is a property combining the afore-mentioned self-healing properties and strength or rigidity.
Hitherto, as films for the decorative plates, vinyl chloride resin films have been most generally used. The vinyl chloride resin films have also been applied to many decorative plates laminated by clear film. The vinyl chloride resin films are readily deformable, and therefore, show a good suitability for V-cut processing. However, since the vinyl chloride resin films are deteriorated in resistance to impact deformation due to lack of appropriate mechanical strength, there arises a problem in which the thickness of the film must be sufficiently increased, or the like.
In addition, since the vinyl chloride resin films cause environmental problems upon burning, there is a strong demand for developing films comprising other materials instead of the vinyl chloride resin films. Further, when the vinyl chloride resin film is adhered to a substrate through an adhesive layer, there arises such a problem that a plasticizer or a stabilizer contained in vinyl chloride resin is transferred to the adhesive layer, resulting in deteriorated adhesion thereof to the substrate. Furthermore, since the vinyl chloride resin films inherently show large elongation or shrinkage upon heating due to low thermal dimensional stability thereof, there also arises a problem in which the decorative plate laminated by the vinyl chloride resin film is likely to suffer from wrinkles on a surface thereof.
Incidentally, with respect to decorative plates using other films than the vinyl-chloride resin films, there have been proposed a decorative plate in which a laminated material comprising a polyolefin-based synthetic paper as base film or a flexible polyester film as a base film is used (Japanese Patent Application Laid-Open (KOKAI) No. 7-17005 (1995)); a decorative plate in which a laminated material comprising a thermoplastic transparent elastomer film selected from the group consisting of styrene-based films, olefin-based films, urethane-based films, fluororubber-based films, polyamide-based films and ester-based films, or an polyolefin-based resin is used (Japanese Patent Application Laid-Open (KOKAI) No. 6-79830 (1995)); or the like.
However, the afore-mentioned conventional decorative plates using the polyolefin-based resin films are not suitable for furniture or interior building materials, because these decorative plates show a large calorific value upon burning, e.g., upon fire. For this reason, there is a demand for decorative plates using other films than the polyolefin-based resin films.
Polyester films are advantageously used for the decorative plates because of relatively low calorific value and less environmental problem upon burning thereof. Further, the polyester films are excellent in thermal dimensional stability. However, the polyester films are usually deteriorated in the suitability for V-cut processing. In addition, in many cases, the polyester films are unsatisfactory in adhesion to plywood. Also, when it is intended to form a picture printed layer on the polyester film, the adhesion thereto is unsatisfactory, because the polyester films are deteriorated in adhesion to an adhesive layer or the picture printed layer. Accordingly, there are conventionally known no decorative plates laminated by clear film, which are excellent in suitability for V-cut processing, in resistance to impact deformation and in adhesion property, and which cause less environmental pollution upon burning.
As a result of the present inventors' earnest studies for solving the above problems, it has been found that by using a polyester film coated on at least one surface thereof with a coating layer and having a specific optical density, a specific coefficient of thermal shrinkage and a specific tensile modulus, as a film layer of a decorative plate in which at least the film layer and a picture printed layer are in turn laminated over a surface of a substrate, or as an inner film layer of a decorative plate laminated by clear film in which at least a first adhesive layer, the inner film layer, a picture printed layer, a second adhesive layer and an outer film layer are in turn laminated over a substrate, there can be obtained the decorative plate or the decorative plate laminated by clear film which is free from the afore-mentioned problems and can show excellent properties. The present invention has been attained on the basis of the above finding.